1. Field of the Invention
This invention relates to a multi-stage flash distillation plant.
2. Description of the Prior Art
The prior art multi-stage flash distillation plant consists essentially of: a flash evaporator composed of a heat recovery section having several stage flash chambers and a heat rejection section having at least two stage flash chambers; a deaerator, thickener, seed mixing tank, brine recycling pump, brine extracting pump, brine heater, and the like. The cold sea water to be supplied to the heat rejection section is fed from the final flash chamber side through the deaerator, while part of the cold sea water thus supplied is discharged to the exterior and the remaining part thereof is supplied as feed sea water to the rear chamber of the final stage flash chamber. Simultaneously therewith, the seed-brine mixture is supplied from the seed-mixing tank to the rear chamber of the final stage flash chamber. The feed sea water and seed-mixed brine are mixed with the concentrated brine which has been fed into the rear chamber of the final stage flash chamber, and then pressurized by the brine circulating pump and fed through a condenser provided in the heat recovery section from the rear stage flash chamber in the heat recovery section, then through a first flash chamber into the brine heater for being heated therein, then into the first stage flash chamber for flash evaporation, through the succeeding flash chambers sequentially to give a concentrated brine and then into the final flash chamber. The concentrated brine present in the front chamber of the final stage flash chamber is fed by means of a brine extracting pump to the thickener, wherein the concentrated brine is separated into clean brine containing little seeds and brine containing a relatively great amount of seeds. The brine containing a relatively great amount of seeds is pressurized by means of a slurry circulating pump by way of a down-flow pipe attached to the lower portion of the thickener and introduced to the discharge side of the flash circulating pump and then into the condenser in the final flash chamber of a heat recovery section. As a result, if the thickener is open to the atmosphere, then an air will be dissolved into the brine, thereby accelerating corrosion in the brine circulating system. For this reason, it is required that the thickener be prepared in air-tight fashion and nitrogen gas be fed into the thickener from a nitrogen bomb.
On the other hand, since air is possibly dissolved into the brine at the time of mixing seeds with brine in the seed-mixing tank, the seed-brine mixture is discharged from the distributor open in the rear chamber of the final stage flash chamber, into the aforesaid rear chamber for deaeration. However, there arises, in this respect, a disadvantage in that if a seed-mixed brine having a high seed concentration is supplied to the distributor, then there will be encountered clogging of the nozzle of the distributor.
As has been described thus far, with the conventional multi-stage flash distillation plant, the brine from the downflow pipe of the thickener is fed to the discharge side of the brine circulating pump, so that the pressure of brine should be increased. For this reason, a slurry circulating pump is provided, and nitrogen gas is sealingly filled in the thickener so as to prevent the dissolution of air into the brine. Thus, this adds to the cost of the water produced.
Furthermore, it remains desired that the deaerator for deaerating the feed sea water be omitted and, in addition that, the seed concentration of the seed-brine mixture to be supplied to the distributor is reduced.
Accordingly, it is the first object of the present invention to provide a multi-stage flash distillation plant which dispenses with a slurry circulating pump and the need to sealingly fill nitrogen gas the thickener.
It is the second object of the present invention to provide a multi-stage flash destillation plant which dispenses with a deaerator for deaerating feed sea water and which prevents clogging of the nozzle of a distributor by lowering the seed concentration of the seed-brine mixture.
These and other objects and advantages of the present invention will become fully apparent from the following description when read in conjunction with the accompanying drawings.